Methods and apparatus for cleaning substrates

ABSTRACT

The present invention discloses a method for cleaning substrate without damaging patterned structure on the substrate using ultra/mega sonic device, comprising: applying liquid into a space between a substrate and an ultra/mega sonic device; setting an ultra/mega sonic power supply at frequency f1 and power P1 to drive said ultra/mega sonic device; after micro jet generated by bubble implosion and before said micro jet generated by bubble implosion damaging patterned structure on the substrate, setting said ultra/mega sonic power supply at frequency f2 and power P2 to drive said ultra/mega sonic device; after temperature inside bubble cooling down to a set temperature, setting said ultra/mega sonic power supply at frequency f1 and power P1 again; repeating above steps till the substrate being cleaned.

FIELD OF THE INVENTION

The present invention generally relates to method and apparatus for cleaning substrate. More particularly, relates to controlling the bubble cavitation generated by ultra or mega sonic device during the cleaning process to achieve a stable or controlled cavitation on the entire substrate, which removes fine particles efficiently without damaging the device structure on the substrate.

BACKGROUND

Semiconductor devices are manufactured or fabricated on semiconductor wafers using a number of different processing steps to create transistor and interconnection elements. Recently, the transistors are built from two dimensions to three dimensions such as finFET transistors and 3D NAND memory. To electrically connect transistor terminals associated with the semiconductor wafer, conductive (e.g., metal) trenches, vias, and the like are formed in dielectric materials as part of the semiconductor device. The trenches and vias couple electrical signals and power between transistors, internal circuit of the semiconductor devices, and circuits external to the semiconductor device.

In forming the finFET transistors and interconnection elements on the semiconductor wafer may undergo, for example, masking, etching, and deposition processes to form the desired electronic circuitry of the semiconductor devices. In particular, multiple masking and plasma etching step can be performed to form a pattern of finFET, 3D NAND flash cell and or recessed areas in a dielectric layer on a semiconductor wafer that serve as fin for the transistor and or trenches and vias for the interconnection elements. In order to removal particles and contaminations in fin structure and or trench and via post etching or photo resist ashing, a wet cleaning step is necessary. Especially, when device manufacture node migrating to 14 or 16 nm and beyond, the side wall loss in fin and or trench and via is crucial for maintaining the critical dimension. In order to reduce or eliminate the side wall loss, it is important to use moderate, dilute chemicals, or sometime de-ionized water only. However, the dilute chemical or de-ionized water usually is not efficient to remove the particles in the fin structure, 3D NAND hole and or trench and via. Therefore the mechanical force such as ultra or mega sonic is needed in order to remove those particles efficiently. Ultra sonic or mega sonic wave will generate bubble cavitation which applies mechanical force to wafer structure, the violent cavitation such as transit cavitation or micro jet will damage those patterned structures. To maintain a stable or controlled cavitation is key parameters to control the mechanical force within the damage limit and at the same time efficiently to remove the particles. In the 3D NAND hole structure, the transit cavitation may not damage the hole structure, but however, the cavitation saturated inside hole will stop or reduce the cleaning effects.

Mega sonic energy coupled with nozzle to clean semiconductor wafer is disclosed in U.S. Pat. No. 4,326,553. The fluid is pressurized and mega sonic energy is applied to the fluid by a mega sonic transducer. The nozzle is shaped to provide a ribbon-like jet of cleaning fluid vibrating at ultra/mega sonic frequencies for the impingement on the surface.

A source of energy vibrates an elongated probe which transmits the acoustic energy into the fluid is disclosed in U.S. Pat. No. 6,039,059. In one arrangement, fluid is sprayed onto both sides of a wafer while a probe is positioned close to an upper side. In another arrangement, a short probe is positioned with its end surface close to the surface, and the probe is moved over the surface as wafer rotates.

A source of energy vibrates a rod which rotates around it axis parallel to wafer surface is disclosed in U.S. Pat. No. 6,843,257 B2. The rod surface is etched to curve groves, such as spiral groove.

It is needed to have a better method for controlling the bubble cavitation generated by ultra or mega sonic device during the cleaning process to achieve a stable or controlled cavitation on the entire wafer, which removes fine particles efficiently without damaging the device structure on the wafer.

SUMMARY

One method of the present invention is to achieve a damage free ultra/mega-sonic cleaning on a wafer with patterned structure by maintaining a stable bubble cavitation. The stable bubble cavitation is controlled by setting a sonic power supply with power P₁ at a time interval shorter than τ₁, and setting the sonic power supply with power P₂ at a time interval longer than τ₂, and repeat above steps till the wafer is cleaned, where power P₂ is equal to zero or much smaller than power P₁, τ₁ is a time interval that the temperature inside bubble raises to a critical implosion temperature; and τ₂ is a time interval that the temperature inside bubble falls down to a temperature much lower than the critical implosion temperature.

Another method of the present invention is to achieve a damage free ultra/mega sonic cleaning on a wafer with patterned structure by maintaining a stable bubble cavitation. The stable bubble cavitation is controlled by setting a sonic power supply with frequency f₁ at a time interval shorter than τ₁, and setting the sonic power supply with frequency f₂ at a time interval longer than τ₂, and repeat above steps till the wafer is cleaned, where f₂ is much higher than f₁, better to be 2 times or 4 times higher, τ₁ is a time interval that the temperature inside bubble raises to a critical implosion temperature; and τ₂ is a time interval that the temperature inside bubble falls down to a temperature much lower than the critical implosion temperature.

Another method of the present invention is to achieve a damage free ultra/mega-sonic cleaning on a wafer with patterned structure by maintaining a stable bubble cavitation with bubble size less than space in patterned structure. The stable bubble cavitation with bubble size less than space in patterned structure is controlled by setting a sonic power supply at power P₁ for a time interval shorter than τ₁, and setting the sonic power supply at power P₂ for a time interval longer than τ₂, and repeat above steps till the wafer is cleaned, where P₂ is equal to zero or much smaller than P₁, τ₁ is a time interval that the bubble size increases to a critical size equal to or larger than the space in patterned structures; and τ₂ is a time interval that the bubble size decreases to a value much smaller than the space in patterned structure.

Another method of the present invention is to achieve a damage free ultra/mega-sonic cleaning on a wafer with patterned structure by maintaining a stable bubble cavitation with bubble size less than space in patterned structure. The stable bubble cavitation with bubble size less than space in patterned structure is controlled by setting a sonic power supply with frequency f₁ for a time interval shorter than τ₁, and setting the sonic power supply with frequency f₂ for a time interval longer than τ₂, and repeat above steps till the wafer is cleaned, where f₂ is much higher than f₁, better to be 2 times or 4 times higher, τ₁ is a time interval that the bubble size increases to a critical size equal to or larger than the space in patterned structures; and τ₂ is a time interval that the bubble size decreases to a value much smaller than the space in patterned structure.

A method of the present invention is to achieve a damage free ultra/mega-sonic cleaning on a wafer with patterned structure by maintaining a controlled transit cavitation. The controlled transit cavitation is controlled by setting a sonic power supply with power P₁ at a time interval shorter than and setting the sonic power supply with power P₂ at a time interval longer than τ₂, and repeat above steps till the wafer is cleaned, where power P₂ is equal to zero or much smaller than power P₁, τ₁ is a time interval that the temperature inside bubble raises higher than a critical implosion temperature; and τ₂ is a time interval that the temperature inside bubble falls down to a temperature much lower than the critical implosion temperature. The controlled transit cavitation will provide the higher PRE (particle removal efficiency) with minimized damage to patterned structures.

Another method of the present invention is to achieve a damage free ultra/mega sonic cleaning on a wafer with patterned structure by maintaining controlled transit cavitation. The controlled transit cavitation is controlled by setting a sonic power supply with frequency f₁ at a time interval shorter than τ₁, and setting the sonic power supply with frequency f₂ at a time interval longer than τ₂, and repeat above steps till the wafer is cleaned, where f₂ is much higher than f₁, better to be 2 times or 4 times higher, τ₁ is a time interval that the temperature inside bubble raises higher than a critical implosion temperature; and τ₂ is a time interval that the temperature inside bubble falls down to a temperature much lower than the critical implosion temperature. The controlled transit cavitation will provide the higher PRE (particle removal efficiency) with minimized damage to patterned structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B depict an exemplary wafer cleaning apparatus using ultra/mega sonic device;

FIGS. 2A-2G depict variety of shape of ultra/mega sonic transducers;

FIG. 3 depicts bubble cavitation during wafer cleaning process;

FIGS. 4A-4B depict a transit cavitation damaging patterned structure on a wafer during cleaning process;

FIGS. 5A-5C depict thermal energy variation inside bubble during cleaning process;

FIGS. 6A-6C depict an exemplary wafer cleaning method;

FIGS. 7A-7C depict another exemplary wafer cleaning method;

FIGS. 8A-8D depict another exemplary wafer cleaning method;

FIGS. 9A-9D depict another exemplary wafer cleaning method;

FIGS. 10A-10B depict another exemplary wafer cleaning method;

FIGS. 11A-11B depict another exemplary wafer cleaning method;

FIGS. 12A-12B depict another exemplary wafer cleaning method;

FIGS. 13A-13B depict another exemplary wafer cleaning method;

FIGS. 14A-14B depict another exemplary wafer cleaning method;

FIGS. 15A-15C depict a stable cavitation damaging patterned structure on a wafer during cleaning process;

FIG. 16 depicts another exemplary wafer cleaning apparatus using ultra/mega sonic device; and

FIG. 17 depicts an exemplary wafer cleaning apparatus using ultra/mega sonic device;

FIGS. 18A-18C depict another exemplary wafer cleaning method;

FIG. 19 depicts another exemplary wafer cleaning method;

FIGS. 20A-20D depict bubble implosion occurs during the cleaning process without damage to patterned structures;

FIGS. 21A-21B depict another exemplary wafer cleaning method.

DETAILED DESCRIPTION

FIGS. 1A to 1B show a wafer cleaning apparatus using a ultra/mega sonic device. The wafer cleaning apparatus consists of wafer 1010, wafer chuck 1014 being rotated by rotation driving mechanism 1016, nozzle 1012 delivering cleaning chemicals or de-ionized water 1032, and ultra/mega sonic device 1003 and ultra/mega sonic power supply. The ultra/mega sonic device 1003 further consists of piezoelectric transducer 1004 acoustically coupled to resonator 1008. Transducer 1004 is electrically excited such that it vibrates and the resonator 1008 transmits high frequency sound energy into liquid. The bubble cavitation generated by the ultra/mega sonic energy oscillates particles on wafer 1010. Contaminants are thus vibrated away from the surfaces of the wafer 1010, and removed from the surfaces through the flowing liquid 1032 supplied by nozzle 1012.

FIGS. 2A to 2G show top view of ultra/mega sonic devices according to the present invention. Ultra/mega sonic device 1003 shown in FIG. 1 can be replaced by different shape of ultra/mega sonic devices 3003, i.e. triangle or pie shape as shown in FIG. 2A, rectangle as shown in FIG. 2B, octagon as shown in FIG. 2C, elliptical as shown in FIG. 2D, half circle as shown in FIG. 2E, quarter circle as shown in FIG. 2F, and circle as shown in FIG. 2G.

FIG. 3 shows a bubble cavitation during compression phase. The shape of bubbler is gradually compressed from a spherical shape A to an apple shape G, finally the bubble reaches to an implosion status I and forms a micro jet. As shown in FIGS. 4A and 4B, the micro jet is very violent (can reaches a few thousands atmospheric pressures and a few thousands ° C.), which can damage the fine patterned structure 4034 on the semiconductor wafer 4010, especially when the feature size t shrinks to 70 nm and smaller.

FIGS. 5A to 5C show simplified model of bubble cavitation according to the present invention. As sonic positive pressure acting on the bubble, the bubble reduces its volume. During this volume shrinking process, the sonic pressure P_(M) did a work to the bubble, and the mechanical work converts to thermal energy inside the bubble, therefore temperature of gas and/or vapor inside bubble increases.

The idea gas equation can be expressed as follows:

p ₀ v ₀ /T ₀ =pv/T  (1),

where, p₀ is pressure inside bubbler before compression, v₀ initial volume of bubble before compression, T₀ temperature of gas inside bubbler before compression, p is pressure inside bubbler in compression, v volume of bubble in compression, T temperature of gas inside bubbler in compression.

In order to simplify the calculation, assuming the temperature of gas is no change during the compression or compression is very slow and temperature increase is cancelled by liquid surrounding the bubble. So that the mechanical work w_(m) did by sonic pressure P_(M) during one time of bubbler compression (from volume N unit to volume 1 unit or compression ratio=N) can be expressed as follows:

$\begin{matrix} {w_{m} = {{\int_{0}^{{x0} - 1}{pSdx}} = {{\int_{0}^{{x0} - 1}{\left( {{S\left( {x_{0}p_{0}} \right)}/\left( {x_{0^{-}}x} \right)} \right){dx}}} = {{{Sx}_{0}p_{0}{\int_{0}^{{x0} - 1}{{dx}/\left( {x_{0^{-}}x} \right)}}} = {{{{- {Sx}_{0}}p_{0}{\ln\left( {x_{0} - x} \right)}}|_{0}^{{x0} - 1}} = {{Sx}_{0}p_{0}{\ln\left( x_{0} \right)}}}}}}} & (2) \end{matrix}$

Where, S is area of cross section of cylinder, x₀ the length of the cylinder, p₀ pressure of gas inside cylinder before compression. The equation (2) does not consider the factor of temperature increase during the compression, so that the actual pressure inside bubble will be higher due to temperature increase. Therefore the actual mechanical work conducted by sonic pressure will be larger than that calculated by equation (2). If assuming all mechanical work did by sonic pressure is partially converted to thermal energy and partially converted mechanical energy of high pressure gas and vapor inside bubble, and such thermal energy is fully contributed to temperature increase of gas inside of bubbler (no energy transferred to liquid molecules surrounding the bubble), and assuming the mass of gas inside bubble staying constant before and after compression, then temperature increase ΔT after one time of compression of bubble can be expressed in the following formula:

ΔT=Q/(mc)=βw _(m)/(mc)=βSx ₀ p ₀ln(x ₀)/(mc)  (3)

where, Q is thermal energy converted from mechanical work, β ratio of thermal energy to total mechanical works did by sonic pressure, m mass of gas inside the bubble, c gas specific heat coefficient. Substituting β=0.65, S=1E-12 m², x₀=1000 μm=1E-3 m (compression ratio N=1000), p₀=1 kg/cm²=1E4 kg/m², m=8.9E-17 kg for hydrogen gas, c=9.9E3 J/(kg ° k) into equation (3), then ΔT=50.9° k. The temperature T₁ of gas inside bubbler after first time compression can be calculated as

T ₁ =T ₀ +ΔT=20° C.+50.9° C.=70.9° C.  (4)

When the bubble reaches the minimum size of 1 micron as shown in FIG. 5B. At such a high temperature, of cause some liquid molecules surrounding bubble will evaporate. After then, the sonic pressure become negative and bubble starts to increase its size. In this reverse process, the hot gas and vapor with pressure P_(G) will do work to the surrounding liquid surface. At the same time, the sonic pressure P_(M) is pulling bubble to expansion direction as shown in FIG. 5C, therefore the negative sonic pressure P_(M) also do partial work to the surrounding liquid too. As the results of the joint efforts, the thermal energy inside bubble cannot be fully released or converted to mechanical energy, therefore the temperature of gas inside bubble cannot cool down to original gas temperature T₀ or the liquid temperature. After the first cycle of cavitation finishes, the temperature T₂ of gas in bubble will be somewhere between T₀ and T₁ as shown in FIG. 6B. Or T₂ can be expressed as

T ₂ =T1−δT=T ₀ +ΔT−δT  (5)

Where δT is temperature decrease after one time of expansion of the bubble, and δT is smaller than ΔT.

When the second cycle of bubble cavitation reaches the minimum bubble size, the temperature T3 of gas and or vapor inside bubbler will be

T3=T2+ΔT=T ₀ +ΔT−δT+ΔT=T ₀+2ΔT−δT  (6)

When the second cycle of bubble cavitation finishes, the temperature T4 of gas and/or vapor inside bubbler will be

T4=T3−δT=T ₀+2ΔT−δT−δT=T ₀+2ΔT−2δT  (7)

Similarly, when the nth cycle of bubble cavitation reaches the minimum bubble size, the temperature T_(2n-1) of gas and or vapor inside bubbler will be

T _(2n-1) =T ₀ +nΔT−(n−1)δT  (8)

When the nth cycle of bubble cavitation finishes, the temperature T_(2n) of gas and/or vapor inside bubbler will be

T _(2n) =T ₀ +nΔT−nδT=T ₀ +n(ΔT−δT)  (9)

As cycle number n of bubble cavitation increase, the temperature of gas and vapor will increase, therefore more molecules on bubble surface will evaporate into inside of bubble 6082 and size of bubble 6082 will increase too, as shown in FIG. 6C. Finally the temperature inside bubble during compression will reach implosion temperature T₁ (normally T₁ is as high as a few thousands ° C.), and violent micro jet 6080 forms as shown in FIG. 6C.

From equation (8), implosion cycle number n_(i) can be written as following:

n _(i)=(T ₁ −T ₀ −ΔT)/(ΔT−δT)+1  (10)

From equation (10), implosion time can be written as following:

$\begin{matrix} {\tau_{i} = {{n_{i}t_{1}} = {{t_{1}\left( {{\left( {T_{i} - T_{0} - {\Delta T}} \right)/\left( {{\Delta T} - {\delta T}} \right)} + 1} \right)} = {{n_{i}/f_{i}} = {\left( {{\left( {T_{i} - T_{0} - {\Delta T}} \right)/\left( {{\Delta T} - {\delta T}} \right)} + 1} \right)/f_{1}}}}}} & (11) \end{matrix}$

Where, t₁ is cycle period, and f₁ frequency of ultra/mega sonic wave.

According to formulas (10) and (11), implosion cycle number n_(i) and implosion time τ_(i) can be calculated. Table 1 shows calculated relationships among implosion cycle number n_(i), implosion time τ_(i) and (ΔT−δT), assuming Ti=3000° C., ΔT=50.9° C., T₀=20° C., f₁=500 KHz, f₁=1 MHz, and f₁=2 MHz.

TABLE 1 ΔT − δT (° C.) 0.1 1 10 30 50 n_(i) 29018 2903 291 98 59 τ_(i) (ms) 58.036 5.806 0.582 0.196 0.118 f₁ = 500 KHz τ_(i) (ms) 29.018 2.903 0.291 0.098 0.059 f₁ = 1 MHz τ_(i) (ms) 14.509 1.451 0.145 0.049 0.029 f₁ = 2 MHz

In order to avoid damage to patterned structure on wafer, a stable cavitation must be maintained, and the bubble implosion or micro jet must be avoided. FIGS. 7A to 7C shows a method to achieve a damage free ultra or mega-sonic cleaning on a wafer with patterned structure by maintaining a stable bubble cavitation according to the present invention. FIG. 7A shows waveform of power supply outputs, and FIG. 7B shows the temperature curve corresponding to each cycle of cavitation, and FIG. 7C shows the bubble size expansion during each cycle of cavitation. Operation process steps to avoid bubble implosion according to the present invention are disclosed as follows:

Step 1: Put ultra/mega sonic device adjacent to surface of wafer or substrate set on a chuck or tank;

Step 2: Fill chemical liquid or gas (hydrogen, nitrogen, oxygen, or CO₂) doped water between wafer and the ultra/mega sonic device;

Step 3: Rotate chuck or oscillate wafer;

Step 4: Set power supply at frequency f₁ and power P_(i);

Step 5: Before temperature of gas and vapor inside bubble reaches implosion temperature T_(i), (or time reach τ₁<τ_(i) as being calculated by equation (11)), set power supply output to zero watts, therefore the temperature of gas inside bubble start to cool down since the temperature of liquid or water is much lower than gas temperature.

Step 6: After temperature of gas inside bubble decreases to room temperature T₀ or time (zero power time) reaches τ₂, set power supply at frequency f₁ and power P₁ again.

Step 7: repeat Step 1 to Step 6 until wafer is cleaned.

In step 5, the time τ₁ must be shorter than in order to avoid bubble implosion, and τ_(i) can be calculated by using equation (11).

In step 6, the temperature of gas inside bubble is not necessary to be cooled down to room temperature or liquid temperature; it can be certain temperature above room temperature or liquid temperature, but better to be significantly lower than implosion temperature T_(i).

According to equations 8 and 9, if (ΔT−δT) can be known, the can be calculated. But in general, (ΔT−δT) is not easy to be calculated or measured directly. The following method can determine the implosion time τ_(i) experimentally.

Step 1: Based on Table 1, choosing 5 different time τ₁ as design of experiment (DOE) conditions,

Step 2: choose time τ₂ at least 10 times of τ₁, better to be 100 times of τ₁ at the first screen test

Step 3: fix certain power P₀ to run above five conditions cleaning on specific patterned structure wafer separately. Here, P₀ is the power at which the patterned structures on wafer will be surely damaged when running on continuous mode (non-pulse mode).

Step 4: Inspect the damage status of above five wafers by SEMS or wafer pattern damage review tool such as AMAT SEM vision or Hitachi IS3000, and then the implosion time τ_(i) can be located in certain range.

Step 1 to 4 can be repeated again to narrow down the range of implosion time τ_(i). After knowing the implosion time τ_(i), the time τ_(i) can be set at a value smaller than 0.5τ_(i) for safety margin. One example of experimental data is described as following.

The patterned structures are 55 nm poly-silicon gate lines. Ultra/mega sonic wave frequency was 1 MHz, and ultra/mega-sonic device manufactured by Prosys was used and operated in a gap oscillation mode (disclosed by PCT/CN2008/073471) for achieving better uniform energy dose within wafer and wafer to wafer. Other experimental parameters and final pattern damage data are summarized in Table 2 as follows:

TABLE 2 CO₂ Process Power Number of conc. Time Density Cycle τ₁ τ₂ Damage Wafer ID (18 μs/cm) (sec) (Watts/cm2) Number (ms) (ms) Sites #1 18 60 0.1 2000 2 18 1216 #2 18 60 0.1 100 0.1 0.9 0

It was clear that the τ₁=2 ms (or 2000 cycle number) introduced as many as 1216 damage sites to patterned structure with 55 nm feature size, but that the τ₁=0.1 ms (or 100 cycle number) introduced zero (0) damage sites to patterned structure with 55 nm feature size. So that the τ_(i) is some number between 0.1 ms and 2 ms, more detail tests need to be done to narrow its range. Obviously, the cycle number related to ultra or mega sonic power density and frequency, the larger the power density, the less the cycle number; and the lower the frequency, the less the cycle number. From above experimental results, we can predict that the damage-free cycle number should be smaller than 2,000, assuming the power density of ultra or mega sonic wave is larger than 0.1 wattsorcm², and frequency of ultra or mega sonic wave is equal to or less than 1 MHz. If the frequency increases to a range larger than 1 MHz or power density is less than than 0.1 watts/cm², it can be predicted that the cycle number will increase.

After knowing the time τ₁, then the time τ₂ can be shorten based on similar DEO method described above, i.e. fix time τ₁, gradually shorten the time τ₂ to run DOE till damage on patterned structure being observed. As the time τ₂ is shorten, the temperature of gas and or vapor inside bubbler cannot be cooled down enough, which will gradually shift average temperature of gas and vapor inside bubbler up, eventually it will trigger implosion of bubble. This trigger time is called critical cooling time. After knowing critical cooling time τ_(c), the time τ₂ can be set at value larger than 2τ_(c) for the same reason to gain safety margin.

FIGS. 8A to 8D show another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 7A, except in step 4 setting ultra/mega sonic power supply at frequency f₁ and power with changing amplitude of waveform. FIG. 8A shows another cleaning method of setting ultra/mega sonic power at frequency f₁ and power with increasing amplitude of waveform in step 4. FIG. 8B shows another cleaning method of setting ultra/mega sonic power supply at frequency f₁ and power with decreasing amplitude of waveform in step 4. FIG. 8C shows another cleaning method of setting ultra/mega sonic power supply at frequency f₁ and power with decreasing first and increasing later amplitude of waveform in step 4. FIG. 8D shows further another cleaning method of setting ultra/mega sonic power at frequency f₁ and power with increasing first and decreasing later amplitude of waveform in step 4.

FIGS. 9A to 9D show another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 7A, except in step 4 setting ultra/mega sonic power supply at changing frequency. FIG. 9A shows another cleaning method of setting ultra/mega sonic power supply at frequency f₁ first then frequency f₃ later, f₁ is higher than f₃ in step 4. FIG. 9B shows another cleaning method of setting ultra/mega sonic power supply at frequency f₃ first then frequency f₁ later, f₁ is higher than f₃ in step 4. FIG. 9C shows another cleaning method of setting ultra/mega sonic power supply at frequency f₃ first, frequency f₁ later and f₃ last, f₁ is higher than f₃ in step 4. FIG. 9D shows another cleaning method of setting ultra/mega sonic power supply at frequency f₁ first, frequency f₃ later and f₁ last, f₁ is higher than f₃ in step 4.

Similar to method shown in FIG. 9C, the ultra/mega sonic power can be set at frequency f₁ first, at frequency f₃ later and at frequency f₄ at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power can be set at frequency f₄ first, at frequency f₃ later and at frequency f₁ at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁

Again similar to method shown in FIG. 9C, the ultra/mega sonic power can be set at frequency f₁ first, at frequency f₄ later and at frequency f₃ at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power can be set at frequency f₃ first, at frequency f₄ later and at frequency f₁ at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power can be set at frequency f₃ first, at frequency f₁ later and at frequency f₄ at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁.

Again similar to method shown in FIG. 9C, the ultra/mega sonic power can be set at frequency f₄ first, at frequency f₁ later and at frequency f₃ at last in step 4, where f₄ is smaller than f₃, and f₃ is smaller than f₁.

FIGS. 10A to 10B show another method to achieve a damage free ultra/mega-sonic cleaning on a wafer with patterned structure by maintaining a stable bubble cavitation in according to the present invention. FIG. 10A shows waveform of power supply outputs, and FIG. 10B shows the temperature curve corresponding to each cycle of cavitation. Operation process steps according to the present invention are disclosed as follows:

Step 1: Put ultra/mega sonic device adjacent to surface of wafer or substrate set on a chuck or tank;

Step 2: Fill chemical liquid or gas doped water between wafer and the ultra/mega sonic device;

Step 3: Rotate chuck or oscillate wafer;

Step 4: Set power supply at frequency f₁ and power P_(i);

Step 5: Before temperature of gas and vapor inside bubble reaches implosion temperature T_(i), (total time τ₁ elapes), set power supply output at frequency f₁ and power P₂, and P₂ is smaller than P₁. Therefore the temperature of gas inside bubble start to cool down since the temperature of liquid or water is much lower than gas temperature.

Step 6: After temperature of gas inside bubble decreases to certain temperature close to room temperature T₀ or time (zero power time) reach τ₂, set power supply at frequency f₁ and power P_(i) again.

Step 7: repeat Step 1 to Step 6 until wafer is cleaned.

In step 6, the temperature of gas inside bubble can not be cooled down to room temperature due to power P₂, there should be a temperature difference ΔT₂ existing in later stage of τ₂ time zone, as shown in FIG. 10B.

FIGS. 11A to 11B show another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 10A, except in step 5 setting ultra/mega sonic power at frequency f₂ and power P₂, where f₂ is lower than f₁ and P₂ is less than P_(i). Since f₂ is lower than f₁, the temperature of gas or vapor inside bubble increasing faster, therefore the P2 should be set significantly less than P1, better to be 5 or 10 times less in order to reduce temperature of gas and or vapor inside bubble.

FIGS. 12A to 12B show another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 10A, except in step 5 setting ultra/mega sonic power at frequency f₂ and power P₂, where f₂ is higher than f₁, and P₂ is equal to P_(i).

FIGS. 13A to 13B show another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 10A, except in step 5 setting ultra/mega sonic power at frequency f₂ and power P₂, where f₂ is higher than f₁, and P₂ is less than P_(i).

FIGS. 14A-14B shows another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 10A, except in step 5 setting ultra/mega sonic power at frequency f₂ and power P₂, where f₂ is higher than f₁, and P₂ is higher than P₁. Since f₂ is higher than f₁, the temperature of gas or vapor inside bubble increasing slower, therefore the P2 can be slightly higher than P1, but must make sure the temperature of gas and vapor inside bubbler decreases in time zone τ₂ comparing to temperature zone τ₁, as shown in FIG. 14B

FIGS. 4A and 4B show that patterned structure is damaged by the violent micro jet. FIGS. 15A and 15B show that the stable cavitation can also damage the patterned structure on wafer. As bubble cavitation continues, the temperature of gas and vapor inside bubble increases, therefore size of bubble 15046 also increases as shown in FIG. 15A. When size of bubble 15048 becomes larger than dimension of space W in patterned structure as shown in FIG. 15B, the expansion force of bubble cavitation can damage the patterned structure 15034 as shown in FIG. 15C. Another cleaning method according to the present invention are disclosed as follows:

Step 1: Put ultra/mega sonic device adjacent to surface of wafer or substrate set on a chuck or tank;

Step 2: Fill chemical liquid or gas doped water between wafer and the ultra/mega sonic device;

Step 3: Rotate chuck or oscillate wafer;

Step 4: Set power supply at frequency f₁ and power P₁;

Step 5: Before size of bubble reaches the same dimension of space Win patterned structures (time τ₁ elapse), set power supply output to zero watts, therefore the temperature of gas inside bubble starts to cool down since the temperature of liquid or water is much lower than gas temperature;

Step 6: After temperature of gas inside bubble continues to reduce (either it reaches room temperature T₀ or time (zero power time) reach τ₂, set power supply at frequency f₁ power P₁ again;

Step 7: repeat Step 1 to Step 6 until wafer is cleaned;

In step 6, the temperature of gas inside bubble is not necessary to be cooled down to room temperature, it can be any temperature, but better to be significantly lower than implosion temperature T_(i). In the step 5, bubble size can be slightly larger than dimension of patterned structures as long as bubble expansion force does not break or damage the patterned structure. The time τ₁ can be determined experimentally by using the following method:

Step 1: Similar to Table 1, choosing 5 different time τ₁ as design of experiment (DOE) conditions,

Step 2: choose time τ₂ at least 10 times of τ₁, better to be 100 times of τ₁ at the first screen test

Step 3: fix certain power P₀ to run above five conditions cleaning on specific patterned structure wafer separately. Here, P₀ is the power at which the patterned structures on wafer will be surely damaged when running on continuous mode (non-pulse mode).

Step 4: Inspect the damage status of above five wafers by SEMS or wafer pattern damage review tool such as AMAT SEM vision or Hitachi IS3000, and then the damage time τ_(i) can be located in certain range.

Step 1 to 4 can be repeated again to narrow down the range of damage time T_(d). After knowing the damage time τ_(d), the time τ₁ can be set at a value smaller than 0.5 τ_(d) for safety margin.

All cleaning methods described from FIG. 7 to FIG. 14 can be applied in or combined with the method described in FIG. 15.

FIG. 16 shows a wafer cleaning apparatus using a ultra/mega sonic device. The wafer cleaning apparatus consists of wafer 16010, wafer chuck 16014 being rotated by rotation driving mechanism 16016, nozzle 16064 delivering cleaning chemicals or de-ionized water 16060, ultra/mega sonic device 16062 coupled with nozzle 16064, and ultra/mega sonic power supply. Ultra/mega sonic wave generated by ultra/mega sonic device 16062 is transferred to wafer through chemical or water liquid column 16060. All cleaning methods described from FIG. 7 to FIG. 15 can be used in cleaning apparatus described in FIG. 16.

FIG. 17 shows a wafer cleaning apparatus using a ultra/mega sonic device. The wafer cleaning apparatus consists of wafers 17010, a cleaning tank 17074, a wafer cassette 17076 holding the wafers 17010 and being held in the cleaning tank 17074, cleaning chemicals 17070, a ultra/mega sonic device 17072 attached to outside wall of the cleaning tank 17074, and a ultra/mega sonic power supply. At least one inlet fills the cleaning chemicals 17070 into the cleaning tank 17074 to immerse the wafers 17010. All cleaning methods described from FIG. 7 to FIG. 15 can be used in cleaning apparatus described in FIG. 17.

FIGS. 18A to 18C show another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 7A, except in Step 5: Before temperature of gas and vapor inside bubble reaches implosion temperature T_(i), (or time reach τ₁<τ_(i) as being calculated by equation (11)), set power supply output to a positive value or negative DC value to hold or stop ultra/mega sonic device to vibrate, therefore the temperature of gas inside bubble start to cool down since the temperature of liquid or water is much lower than gas temperature. The positive value of negative value can be bigger, equal to or smaller than power P₁.

FIG. 19 shows another embodiment of wafer cleaning method using a ultra/mega sonic device according to the present invention. The method is similar to that shown in FIG. 7A, except in Step 5: Before temperature of gas and vapor inside bubble reaches implosion temperature T_(i), (or time reach τ₁<τ_(i) as being calculated by equation (11)), set power supply output at the frequency same as f₁ with reverse phase to f₁ to quickly stop the cavitation of bubble. Therefore the temperature of gas inside bubble start to cool down since the temperature of liquid or water is much lower than gas temperature. The positive value of negative value can be bigger, equal or less than power P₁. During above operation, the power supply output can be set at a frequency different from frequency f₁ with reverse phase to f₁ in order to quickly stop the cavitation of bubble.

FIGS. 20A-20D show bubble implosion occurs during the bubble cavitation generated by the ultra or mega sonic energy without damage patterned structures on the wafer. In some case, the patterned structure such as vias 20034 and trenches 20036 on the wafer 20010 has a certain mechanical strength, such as that the size of bubble cavitation generated by high frequency sonic power is controlled in a rather small dimension which is much smaller than the critical dimension of the patterned structure, therefore, these small bubble implosion force exerts a very small impact to the patterned structure with large dimension, or that the material characteristic of the patterned structure can sustain a certain intensity of mechanical force naturally. If the micro jet force generated by the bubble implosion is controlled within the intensity that the patterned structures can sustain, the patterned structures will not be damaged. In addition, the mechanical force of the micro jet generated by the bubble implosion contributes to the particles or residues removal performance inside the patterned structures of vias and or trenches or on the surface of the wafer, and it also achieves a higher cleaning efficiency. The micro jet with a controllable intensity, higher than the implosion point yet lower than the damage point, is desired in a cleaning process for a better cleaning performance and efficiency.

FIGS. 21A-21B show that during the sonic power P₁ working on the bubbles at the time of τ₁, the bubble implosion starts to occur when the temperature of first bubble reaching to its implosion temperature at the point of T_(i), and then some bubble implosion continues to occur during the temperature increasing from T_(i) to T_(n) (during the time of Δτ), and then turning the sonic power off in the time interval of τ₂, the temperature of the bubble cooling down from T_(n) to original T₀ by the surrounding liquid. T_(i) is determined as a threshold of the temperature for general bubble implosion in the features of vias and trenches, which triggers the first bubble implosion.

Since thermal transfer is not exactly uninform in the features, more and more bubble implosion will keep occurring after the temperature reaching to T_(i). The bubble implosion intensity will become higher and higher while the implosion temperature T increasing. However, the bubble implosion is controlled under the implosion intensity that would result in the patterned structures damage by controlling the temperature T_(n) below the temperature T_(d) (controlling time of Δτ), wherein T_(r), is the bubbles maximum temperature obtained by sonic power keeping working on the bubbles after the cycles of n, and T_(d) is the temperature of the accumulation of certain amount bubbles implosion with a high intensity (or power) to result in the patterned structure damage. In a cleaning process, the control of bubble implosion intensity is achieved by the control of the time Δt after the first bubble implosion start, so as to achieve a desired cleaning performance and efficiency, and prevent the intensity too high to cause the patterned structure damage.

FIGS. 21A-21B show another embodiment of wafer cleaning method using an ultra/mega sonic device according to the present invention. In order to increase particle removal efficiency (PRE), it is desired to have a controlled transit cavitation in the mega-sonic cleaning process. The controlled transit cavitation is reached by setting a sonic power supply with power P₁ at a time interval shorter than τ₁, and setting the sonic power supply with power P₂ at a time interval longer than τ₂, and repeating above steps till the wafer is cleaned, where power P₂ is equal to zero or much smaller than power P₁, τ₁ is a time interval that the temperature inside bubble raises higher than a critical implosion temperature; and τ₂ is a time interval that the temperature inside bubble falls down to a temperature much lower than the critical implosion temperature. Since the controlled transit cavitation will have certain bubble implosion in the cleaning process, therefore the controlled transit cavitation will provide the higher PRE (particle removal efficiency) with minimized damage to patterned structures. The critical implosion temperature is the lowest temperature inside bubble, which will cause the first bubble implosion. In order to further increase PRE, it is needed to further increase temperature of the bubbles, therefore a longer time τ₁ is needed. Also the temperature of bubble can be increased by shorting the time of τ₂. The frequency of ultra or mega sonic is another parameter to control the level of implosion. Normally, the higher the frequency is, the lower level or intensity of the implosion is.

Another embodiment of wafer cleaning method using an ultra/mega sonic device to achieve a damage free ultra/mega sonic cleaning on a wafer with patterned structure by maintaining controlled transit cavitation is provided according to the present invention. The controlled transit cavitation is controlled by setting a sonic power supply with frequency f₁ at a time interval shorter than τ₁, and setting the sonic power supply with frequency f₂ at a time interval longer than τ₂, and repeating above steps till the wafer is cleaned, where f₂ is much higher than f₁, better to be 2 times or 4 times higher, τ₁ is a time interval that the temperature inside bubble raises higher than a critical implosion temperature; and τ₂ is a time interval that the temperature inside bubble falls down to a temperature much lower than the critical implosion temperature. The controlled transit cavitation will provide the higher PRE (particle removal efficiency) with minimized damage to patterned structures. The critical implosion temperature is the lowest temperature inside bubble, which will cause the first bubble implosion. In order to further increase PRE, it is needed to further increase temperature of the bubbles, therefore a longer time τ₁ is needed. Also the temperature of bubble can be increased by shorting the time of τ₂. The frequency of ultra or mega sonic is another parameter to control the level of implosion. Normally, the higher the frequency is, the lower level or intensity of the implosion is.

As described above, the present invention provides a method for cleaning substrate without damaging patterned structure on the substrate using ultra/mega sonic device. The method comprises: applying liquid into a space between a substrate and an ultra/mega sonic device; setting an ultra/mega sonic power supply at frequency f₁ and power P₁ to drive said ultra/mega sonic device; after micro jet generated by bubble implosion and before said micro jet generated by bubble implosion damaging patterned structure on the substrate, setting said ultra/mega sonic power supply at frequency f₂ and power P₂ to drive said ultra/mega sonic device; after temperature inside bubble cooling down to a set temperature, setting said ultra/mega sonic power supply at frequency f₁ and power P₁ again; repeating above steps till the substrate being cleaned.

In an embodiment, the present invention provides an apparatus for cleaning substrate using ultra/mega sonic device. The apparatus includes a chuck, an ultra/mega sonic device, at least one nozzle, an ultra/mega sonic power supply and a controller. The chuck holds a substrate. The ultra/mega sonic device is positioned adjacent to the substrate. The at least one nozzle injects chemical liquid on the substrate and a gap between the substrate and the ultra/mega sonic device. The controller sets the ultra/mega sonic power supply at frequency f₁ and power P₁ to drive said ultra/mega sonic device; after micro jet generated by bubble implosion and before said micro jet generated by bubble implosion damaging patterned structure on the substrate, setting the ultra/mega sonic power supply at frequency f₂ and power P₂ to drive said ultra/mega sonic device; after temperature inside bubble cooling down to a set temperature, setting the ultra/mega sonic power supply at frequency f₁ and power P₁ again; repeating above steps till the substrate being cleaned.

In another embodiment, the present invention provides an apparatus for cleaning substrate using ultra/mega sonic device. The apparatus includes a cassette, a tank, an ultra/mega sonic device, at least one inlet, an ultra/mega sonic power supply and a controller. The cassette holds at least one substrate. The tank holds said cassette. The ultra/mega sonic device is attached to outside wall of said tank. The at least one inlet is used for filling chemical liquid into said tank to immerse said substrate. The controller sets the ultra/mega sonic power supply at frequency f₁ and power P₁ to drive said ultra/mega sonic device; after micro jet generated by bubble implosion and before said micro jet generated by bubble implosion damaging patterned structure on the substrate, setting said ultra/mega sonic power supply at frequency f₂ and power P₂ to drive said ultra/mega sonic device; after temperature inside bubble cooling down to a set temperature, setting said ultra/mega sonic power supply at frequency f₁ and power P₁ again; repeating above steps till the substrate being cleaned.

In another embodiment, the present invention provides an apparatus for cleaning substrate using ultra/mega sonic device. The apparatus includes a chuck, an ultra/mega sonic device, a nozzle, an ultra/mega sonic power supply and a controller. The chuck holds a substrate. The ultra/mega sonic device coupled with the nozzle is positioned adjacent to the substrate. The nozzle injects chemical liquid on the substrate. The controller sets the ultra/mega sonic power supply at frequency f₁ and power P₁ to drive said ultra/mega sonic device; after micro jet generated by bubble implosion and before said micro jet generated by bubble implosion damaging patterned structure on the substrate, setting said ultra/mega sonic power supply at frequency f₂ and power P₂ to drive said ultra/mega sonic device; after temperature inside bubble cooling down to a set temperature, setting said ultra/mega sonic power supply at frequency f₁ and power P₁ again; repeating above steps till the substrate being cleaned.

The embodiments disclosed from FIG. 8 to FIG. 19 can be applied in embodiments disclosed in FIG. 21.

Generally speaking, an ultra/mega sonic wave with the frequency between 0.1 MHz˜10 MHz may be applied to the method disclosed in the present invention.

Although the present invention has been described with respect to certain embodiments, examples, and applications, it will be apparent to those skilled in the art that various modifications and changes may be made without departing from the invention. 

1.-98. (canceled)
 99. An apparatus for cleaning a semiconductor wafer comprising features of patterned structures, the apparatus comprising: a wafer holder configured to hold the semiconductor wafer; an inlet configured to apply liquid on the semiconductor wafer; a transducer configured to deliver acoustic energy to the liquid; a power supply of the transducer; and a controller for the power supply comprising a timer, the controller being configured to control the transducer based on the timer to: deliver acoustic energy to the liquid at a first frequency and a first power level for a predetermined first time period, wherein bubble implosion occurs in the first time period, and deliver acoustic energy to the liquid at a second frequency and a second power level for a predetermined second time period, wherein the controller is configured to alternately apply the first and second time periods one after another for a predetermined number of cycles, wherein the first and second time periods, the first and second power levels, and the first and second frequencies are determined such that no feature is damaged as a result of delivering the acoustic energy.
 100. The apparatus of claim 99, wherein the wafer holder comprises a rotating chuck.
 101. The apparatus of claim 99, wherein the wafer holder comprises a cassette submerged in a cleaning tank.
 102. The apparatus of claim 99, wherein the inlet comprises a nozzle.
 103. The apparatus of claim 99, wherein the transducer is connected to the inlet and imparts acoustic energy to the liquid flowing through the inlet.
 104. The apparatus of claim 99, wherein the second power level is lower than the first power level.
 105. The apparatus of claim 104, wherein the second power level is zero.
 106. The apparatus of claim 99, wherein the second frequency is higher than the first frequency.
 107. The apparatus of claim 99, wherein acoustic energy in the second time period is in antiphase to acoustic energy in the first time period.
 108. The apparatus of claim 99, wherein the first frequency is equal to the second frequency, while the first power level is higher than the second power level.
 109. The apparatus of claim 99, wherein the first frequency is higher than the second frequency, while the first power level is higher than the second power level.
 110. The apparatus of claim 99, wherein the first frequency is lower than the second frequency, while the first power level is equal to the second power level.
 111. The apparatus of claim 99, wherein the first frequency is lower than the second frequency, while the first power level is higher than the second power level.
 112. The apparatus of claim 99, wherein the first frequency is lower than the second frequency, while the first power lever is lower than the second power level.
 113. The apparatus of claim 99, wherein the first power level rises during the first time period.
 114. The apparatus of claim 99, wherein the first power level falls during the first time period.
 115. The apparatus of claim 99, wherein the first power level both rises and falls during the first time period.
 116. The apparatus of claim 99, wherein the first frequency changes from a higher value to a lower value during the first time period.
 117. The apparatus of claim 99, wherein the first frequency changes from a lower value to a higher value during the first time period.
 118. The apparatus of claim 99, wherein the first frequency changes from a lower value to a higher value and then back to the lower value during the first time period.
 119. The apparatus of claim 99, wherein the first frequency changes from a higher value to a lower value and then back to the higher value during the first time period.
 120. The apparatus of claim 99, wherein the first frequency is set as f₁ first, f₃ later and f_(a) at last during the first time period, where f_(a) is smaller than f₃, and f₃ is smaller than f₁.
 121. The apparatus of claim 99, wherein the first frequency is set as f₄ first, f₃ later and f₁ at last during the first time period, where f₄ is smaller than f₃, and f₃ is smaller than f₁.
 122. The apparatus of claim 99, wherein the first frequency is set as f₁ first, f₄ later and f₃ at last during the first time period, where f₄ is smaller than f₃, and f₃ is smaller than f₁.
 123. The apparatus of claim 99, wherein the first frequency is set as f₃ first, f₄ later and f₁ at last during the first time period, where f₄ is smaller than f₃, and f₃ is smaller than f₁.
 124. The apparatus of claim 99, wherein the first frequency is set as f₃ first, f₁ later and f₄ at last during the first time period, where f₄ is smaller than f₃, and f₃ is smaller than f₁.
 125. The apparatus of claim 99, wherein the first frequency is set as f_(a) first, f₁ later and f₃ at last during the first time period, where f_(a) is smaller than f₃, and f₃ is smaller than f₁.
 126. The apparatus of claim 99, wherein the second frequency is zero and the second power level remains a constant positive value during the second time period.
 127. The apparatus of claim 99, wherein the second frequency is zero and the second power level remains a constant negative value during the second time period.
 128. The apparatus of claim 99, wherein the features comprise vias or trenches having depth to width ratios of at least
 3. 129. The apparatus of claim 99, wherein a device manufacturing node of the semiconductor wafer is no more than 16 nanometers.
 130. The apparatus of claim 99, wherein the wafer holder is further configured to rotate the wafer with respect to the transducer as acoustic energy is delivered.
 131. The apparatus of claim 99, wherein the bubble implosion is controlled under an implosion intensity that would result in damages to the patterned structures by controlling Tn below Td, wherein Tn is a maximum temperature of the bubbles after the predetermined number of cycles, and Td is a temperature of the bubbles that would result in implosion with a high intensity damaging the patterned structures.
 132. The apparatus of claim 99, wherein the features are not damaged by expansion of bubbles in the first time period.
 133. The apparatus of claim 99, wherein temperatures inside bubbles decrease in the second time period.
 134. The apparatus of claim 133, wherein temperatures inside the bubble decrease to near a temperature of said liquid in the second time period.
 135. The apparatus of claim 99, wherein the first time period is shorter than 2,000 times of a cycle period of the first frequency.
 136. The apparatus of claim 99, wherein the first time period is shorter than ((T_(i)−T₀−ΔT)/(ΔT−δT)+1)/f₁, where T_(i) is an implosion temperature, T₀ is a temperature of the liquid, ΔT is a temperature increase after one time of compression, ΔT is a temperature decrease after one time of expansion, and f1 is the first frequency.
 137. A controller for a power supply of a transducer comprising a timer, the controller being configured to control the transducer based on the timer to: deliver acoustic energy to liquid applied on a semiconductor wafer at a first frequency and a first power level for a predetermined first time period, wherein bubble implosion occurs in the first time period; and deliver acoustic energy to the liquid at a second frequency and a second power level for a predetermined second time period, wherein the controller is configured to alternately apply the first and second time periods one after another for a predetermined number of cycles, wherein the first and second time periods, the first and second power levels, and the first and second frequencies are determined such that no feature is damaged as a result of delivering the acoustic energy.
 138. The controller of claim 137, wherein the second power level is lower than the first power level.
 139. The controller of claim 138, wherein the second power level is zero. 